364 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 8 



with the establishment of intensive maritime traffic with eastern Asia. 

 A specimen was first brought to me for identification in 1923 and a 

 decade and a half or perhaps two may well have passed after they 

 had first been introduced before we became fully aware of this new 

 inhabitant in German waters. 



It must seem astonishing that the mitten crab has increased within 

 a period of perhaps three and a half decades in the German rivers in 

 such an alarming degree. This tremendous increase was undoubtedly 

 aided by the fact that they were not brought in fully grown but in 

 large numbers as larvae or when very young. However, this enor- 

 mous increase in such a short time was above all aided by the partic- 

 ular conditions in the German rivers. 



The German rivers ceased long ago to be just rivers and became 

 waterways, navigation highways, and traffic arteries. This change 

 of the German rivers has made existence impossible for many native 

 animals, as, for instance, for the predatory fishes which would have 

 been of the greatest importance in fighting and checking these mitten 

 crabs. Conditions were thus created under which this extraordinarily 

 resistant mitten crab has been able to establish itself. 



THE DISTRIBUTION OF THE MITTEN CRAB IN GERMANY 



Although the mitten crab was found first in the Weser River system 

 (in Aller near Rethcm in 1912), it has not been seen there since. By 

 later questioning Elbe fishermen, it was found that the crabs had 

 appeared as a by-catch in the flounder fisheries by the mouth of the 

 Elbe since about 1915. They seem hardly to have left the lower 

 tide-water region after that. They were first seen in the upper tide- 

 water regions above Hamburg in 1926, and by 1927 they were there in 

 great masses. A few years later they flooded the waters of the mid 

 Elbe (Havel, Province of Brandenburg) to such an extent that it 

 became necessary to take measures to check them. By 1930 the lower 

 sections of the Weser, as well as the lower and mid sections of the 

 Elbe, were thickly infested with these crabs. In the late twenties they 

 began to spread westward into the Ems and eastward into the Oder. 

 They found their way westward from the Weser into the Ems through 

 the many streams and canals in northern Oldenburg and eastward into 

 the Oder through the waterways leading from the Elbe through 

 Brandenburg. And with the beginning of this decade they com- 

 menced to spread westward from the Ems into northern Holland and 

 through the Midland Canal and the Rhine into southern Holland, 

 northern Belgium, and northern France. They have gradually estab- 

 lished a particular breeding ground in the region of the Danish Great 

 and Lesser Belt in the Baltic. Mitten crabs that had reached the Oder 

 from the Elbe through Brandenburg, have with all certainty helped 

 to establish these breeding grounds inasmuch as, upon reaching 



